opkg: add --no-check-certificate argument

opkg: add --no-check-certificate argument

For cases when artifacts are stored on https:// accessible
locations and you don't want to install ca-certificates
(for various reasons).

I'll admit, using SSL like this is not recommended,
but since wget (even uclient-fetch) allows the
--no-check-certificate option, it would be nice
for opkg to support setting it if needed/configured.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>